1. Field of the Invention
The present invention relates to a polycrystalline yttria sintered body with good light transmission, particularly a light transmitting yttria sintered body suitable for an infrared transmitting window which has a thickness of not smaller than 3 mm, and a method for preparing the same.
2. Description of the Related Art
Since yttria (Y.sub.2 O.sub.3) has a cubic system crystal structure at a temperature of not higher than 2350.degree. C., it has low scattering at crystal grain boundaries and exhibits good light transmission when sintered at a high density.
Usually, light transmission of the yttria sintered body sharply increases around a wavelength of 0.3 .mu.m in a visible region and reaches the maximum value around a wavelength of 3 to 7 .mu.m in an infrared region. Therefore, the yttria sintered body is a promising light transmitting material such as an optical window and is manufactured by various methods.
For example, Japanese Patent Kokai Publication No. 17911/1979 discloses a method which comprises adding lanthanum oxide (La.sub.2 O.sub.3) as a sintering aid and effecting the sintering under a low oxygen atmosphere and Japanese Patent Kokai Publication No. 17910/1979 discloses a method which comprises using alumina as a sintering agent and effecting the sintering under a low oxygen atmosphere. Further, U.S. Pat. No. 3,878,280 discloses a method which comprises hot pressing yttria powder under reduced pressure.
However, in the conventional methods for preparing the light transmitting yttria sintered body by using such sintering aid, a second phase tends to form partially due to the presence of the sintering aid such as La.sub.2 O.sub.3 which is added for densifying the sintered body, the transmission decreases by the scattering of the light due to ununiformity of the texture and an optically uniform block of a large size cannot be obtained. In hot pressing under vacuum, since it is difficult to apply high pressure of not lower than 500 kg/cm.sup.2 because of the strength of a graphite mold, sufficient densification cannot proceed and then the transmission is low.
In the light transmitting yttria sintered bodies prepared by these conventional methods, a straight transmission is at most about 80% at a wavelength of 2 to 6 .mu.m when a sample thickness is 2.5 mm. In order to use the yttria sintered body as the material for an infrared transmitting window which usually has a thickness of not smaller than 3 mm, it is necessary to increase the straight transmission.